1. Field of the Invention
This invention relates to error correcting codes.
2. Description of Related Art
Turbo codes are binary error-correcting codes built from the parallel concatenation of two recursive systematic convolutional codes and using a feedback decoder. Recently introduced be Berrou, et al. ("Near Shannon limit error-correcting coding and decoding: Turbo-codes", ICC'93, Conf Rec. pp. 1064-1070, Geneva, May 1993), the basics of such codes are described further in U.S. Pat. Nos. 5,446,747 and 5,406,570.
The reference and patents to Berrou describe a basic turbo code encoder architecture of the type shown in the block diagram in FIG. 1. As described in Berrou '747, FIG. 1 shows a block diagram of a coder in an example where two distinct codes are used in parallel. Each source data element d to be coded is coupled to a first systematic coding module 11 and, through a temporal interleaving module 12, to a second systematic coding module 13. The coding modules 11 and 13 may be of any known systematic type, such as convolutional coders, that take into account at least one of the preceding source data elements in order to code the source data element d. The codes implemented in coding modules 11 and 13 may be identical or different.
The input information bits d feed the first coding module 11 and, after being scrambled by the interleaving module 12, enter the second coding module 13. A codeword of a parallel concatenated code consists of the information input bits to the first encoder followed by the parity check bits of both encoders.
Under this architecture, there are at least two coded data elements Y.sub.1 and Y.sub.2, coming from distinct coders 11 and 13, associated with each source data element d. A data element X, equal to the source data element d, is also transmitted. This characteristic was described in Berrou '747 as "necessary for the making of the decoding modules".
The transmitted coded data elements and source data element become received data elements at a decoder. The task of the decoder is to re-construct the original data source d bit stream from the received data elements, which may have been corrupted by noise.
Thus, an important aspect of prior art turbo code encoders is that they transmit a data element X equal to input source data element d.
The present invention results from observation that the prior art fails to achieve a simpler architecture for the encoder, and fails to provide as robust encoding as is required or desired in certain environments, including low-power, constrained-bandwidth uses, such as deep space communications and personal communication devices, and high-noise environments.